Metal-oxide-semiconductor diodes containing C_(60) fullerenes for non-volatile memory applications

摘要

For non-volatile memories, silicon-oxide-nitride-oxide-silicon or floating gate structures are used to store information by charging and discharging electronic states reversibly. In this article, we propose to replace the floating gate by C_(60) molecules. This would allow more defined programming voltages because of the discrete molecular energy levels and a higher resistance to tunneling oxide defects because of the weak electrical connection between the single molecules. Such C_(60) MOS diode structures are produced and their electrical properties are analyzed regarding current transport and charging mechanism of the molecules. To create the MOS structures, C_(60) molecules (5% of a monolayer) are evaporated onto a part of a clean silicon wafer and covered by amorphous silicon in situ in an ultra high vacuum system. Then the wafer is oxidized in wet atmosphere at just 710 ℃ through the C_(60) layer. The goal is to produce a clean oxide above and under the molecules without destroying them. Aluminum gate contacts are defined on top of these layers to perform complementary capacitance voltage (CV) and current voltage (IV) measurements. First, the gate voltage is swept to analyze the injection current, then CV measurements are performed after each sweep to analyze the charge state of the C_(60) layer and the oxide quality. Reference diodes without C_(60) on the same wafer show an identical Fowler-Nordheim (FN) tunneling behavior for currents injected from silicon or from aluminum, respectively. In the CV curves, no pronounced flatband voltage shift is observable. In diodes with C_(60), for negative gate voltages, a classical FN tunneling is observed and compared to theory. The electron injection from silicon shows a different tunneling current behavior. It starts at a lower electric field and has a smaller slope then a FN current would have. It is identified as a trap-assisted tunneling (TAT) current caused by oxidation-induced traps under the C_(60) layer. It is modeled by an established analytic TAT model which reproduces the data with a trap energy of 1.8 eV below the oxide conduction band. In the CV measurements the negative voltage IV sweeps result in a shift of the flatband voltages to more negative values. Positive voltage IV sweeps shift the CV curves back onto the starting curves. This proves positive charge trapping in the oxide which results in a non-volatile memory behavior for the diodes with C_(60).